/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.codec;

import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.util.Collections;
import java.util.List;
import java.util.Map;

import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

import org.springframework.core.ResolvableType;
import org.springframework.core.codec.CodecException;
import org.springframework.core.codec.Decoder;
import org.springframework.core.codec.StringDecoder;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferFactory;
import org.springframework.core.io.buffer.DefaultDataBufferFactory;
import org.springframework.http.MediaType;
import org.springframework.http.ReactiveHttpInputMessage;
import org.springframework.lang.Nullable;

/**
 * Reader that supports a stream of {@link ServerSentEvent ServerSentEvents} and also plain
 * {@link Object Objects} which is the same as an {@link ServerSentEvent} with data only.
 *
 * @author Sebastien Deleuze
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public class ServerSentEventHttpMessageReader implements HttpMessageReader<Object> {

    private static final DataBufferFactory bufferFactory = new DefaultDataBufferFactory();

    private static final StringDecoder stringDecoder = StringDecoder.textPlainOnly();

    private static final ResolvableType STRING_TYPE = ResolvableType.forClass(String.class);


    @Nullable
    private final Decoder<?> decoder;


    /**
     * Constructor without a {@code Decoder}. In this mode only {@code String}
     * is supported as the data of an event.
     */
    public ServerSentEventHttpMessageReader() {
        this(null);
    }

    /**
     * Constructor with JSON {@code Decoder} for decoding to Objects.
     * Support for decoding to {@code String} event data is built-in.
     */
    public ServerSentEventHttpMessageReader(@Nullable Decoder<?> decoder) {
        this.decoder = decoder;
    }


    /**
     * Return the configured {@code Decoder}.
     */
    @Nullable
    public Decoder<?> getDecoder() {
        return this.decoder;
    }

    @Override
    public List<MediaType> getReadableMediaTypes() {
        return Collections.singletonList(MediaType.TEXT_EVENT_STREAM);
    }

    @Override
    public boolean canRead(ResolvableType elementType, @Nullable MediaType mediaType) {
        return (MediaType.TEXT_EVENT_STREAM.includes(mediaType) || isServerSentEvent(elementType));
    }

    private boolean isServerSentEvent(ResolvableType elementType) {
        Class<?> rawClass = elementType.getRawClass();
        return (rawClass != null && ServerSentEvent.class.isAssignableFrom(rawClass));
    }


    @Override
    public Flux<Object> read(ResolvableType elementType, ReactiveHttpInputMessage message,
                             Map<String, Object> hints) {

        boolean shouldWrap = isServerSentEvent(elementType);
        ResolvableType valueType = (shouldWrap ? elementType.getGeneric() : elementType);

        return stringDecoder.decode(message.getBody(), STRING_TYPE, null, hints)
                .bufferUntil(line -> line.equals(""))
                .concatMap(lines -> buildEvent(lines, valueType, shouldWrap, hints));
    }

    private Mono<?> buildEvent(List<String> lines, ResolvableType valueType, boolean shouldWrap,
                               Map<String, Object> hints) {

        ServerSentEvent.Builder<Object> sseBuilder = shouldWrap ? ServerSentEvent.builder() : null;
        StringBuilder data = null;
        StringBuilder comment = null;

        for (String line : lines) {
            if (line.startsWith("data:")) {
                data = (data != null ? data : new StringBuilder());
                data.append(line.substring(5)).append("\n");
            }
            if (shouldWrap) {
                if (line.startsWith("id:")) {
                    sseBuilder.id(line.substring(3));
                } else if (line.startsWith("event:")) {
                    sseBuilder.event(line.substring(6));
                } else if (line.startsWith("retry:")) {
                    sseBuilder.retry(Duration.ofMillis(Long.valueOf(line.substring(6))));
                } else if (line.startsWith(":")) {
                    comment = (comment != null ? comment : new StringBuilder());
                    comment.append(line.substring(1)).append("\n");
                }
            }
        }

        Mono<?> decodedData = (data != null ? decodeData(data.toString(), valueType, hints) : Mono.empty());

        if (shouldWrap) {
            if (comment != null) {
                sseBuilder.comment(comment.toString().substring(0, comment.length() - 1));
            }
            return decodedData.map(o -> {
                sseBuilder.data(o);
                return sseBuilder.build();
            });
        } else {
            return decodedData;
        }
    }

    private Mono<?> decodeData(String data, ResolvableType dataType, Map<String, Object> hints) {
        if (String.class == dataType.resolve()) {
            return Mono.just(data.substring(0, data.length() - 1));
        }

        if (this.decoder == null) {
            return Mono.error(new CodecException("No SSE decoder configured and the data is not String."));
        }

        byte[] bytes = data.getBytes(StandardCharsets.UTF_8);
        Mono<DataBuffer> input = Mono.just(bufferFactory.wrap(bytes));
        return this.decoder.decodeToMono(input, dataType, MediaType.TEXT_EVENT_STREAM, hints);
    }

    @Override
    public Mono<Object> readMono(ResolvableType elementType, ReactiveHttpInputMessage message,
                                 Map<String, Object> hints) {

        // We're ahead of String + "*/*"
        // Let's see if we can aggregate the output (lest we time out)...

        if (String.class.equals(elementType.getRawClass())) {
            Flux<DataBuffer> body = message.getBody();
            return stringDecoder.decodeToMono(body, elementType, null, null).cast(Object.class);
        }

        return Mono.error(new UnsupportedOperationException(
                "ServerSentEventHttpMessageReader only supports reading stream of events as a Flux"));
    }

}
